Downlink retransmission timing for a Time Division Duplexing (TDD) mode and a Frequency Division Duplexing (FDD) has been specified in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) protocol.
For the FDD mode, answer information transmitted in an uplink (UL) subframe includes Acknowledgement/Negative Acknowledgement (ACK/NACK) information, and it may merely correspond to the data transmission in a downlink (DL) subframe, i.e., in the answer information feedback, one uplink subframe corresponds to one downlink subframe. In addition, in the case that the downlink transmission is detected by a terminal in a downlink subframe n4, the answer information for the downlink subframe n-4 may be fed back in an uplink subframe n. In other words, in the downlink retransmission timing, the uplink subframe n corresponds to a fourth downlink subframe before the uplink subframe n, and it is used to feed back the answer information for this downlink subframe.
For the TDD mode, seven different timeslot configurations have been defined in the 3GPP protocol, i.e., configuration #0 to configuration #6 in Table 1. “D” represents the downlink subframe, “U” represents the uplink subframe, and “S” represents a special subframe. The answer information transmitted in one uplink subframe may correspond to the data transmission in one or more downlink subframes, i.e., in the answer information feedback, one uplink subframe may correspond to one or more downlink subframes. In addition, in the case that the downlink transmission has been detected by the terminal in downlink subframe n-k, the answer information for downlink subframe n-k will be fed back in uplink subframe n. In other words, in the downlink retransmission timing mode, the uplink subframe n corresponds to a kth downlink subframe before uplink subframe n, and it is used to feed back the answer information for this downlink subframe. Table 2 shows a value of k corresponding to each uplink subframe for feeding back the answer information with respect to the different TDD timeslot configurations.
TABLE 1TDD Timeslot ConfigurationsDownlink-Uplink-to-UplinkDownlinkSwitch-Configura-PointSubframe NumbertionPeriodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
TABLE 2Downlink Retransmission Timing Modes for TDD Timeslot ConfigurationsUplink-DownlinkSubframe NumberConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5,——————4, 75——13, 12, 9, 8, 7,———————5, 4, 11, 66——775——77—
Taking configuration #0 in Table 1 as an example, the uplink subframes include subframes 2 to 4, and subframes 7 to 9. As shown in Table 2, Subframes 2, 4, 7 and 9 are used to feed back the answer information. Subframe 2 corresponds to a sixth subframe before it, subframe 4 corresponds to a fourth subframe before it, subframe 7 corresponds to a sixth subframe before it, and subframe 9 corresponds to a fourth subframe before it.
Carrier aggregation has also been defined in the LTE protocol, i.e., more than two and less than five independent carriers may be aggregated by the terminal, so as to enhance a peak rate. For the carriers in the carrier aggregation, a primary carrier may be configured by a system for the terminal, and the other carriers may be taken as secondary carriers.
In the carrier aggregation, the answer information for the downlink subframes of the primary carrier and the secondary carriers may be fed back over the primary carrier. For the FDD carrier aggregation, a downlink data channel retransmission is similar to that for a single carrier. For the TDD carrier aggregation, in the case that the primary carrier has a timeslot configuration identical to the secondary carriers, the downlink data channel retransmission mode is similar to that for the single carrier. In the case that the primary carrier has a timeslot configuration different from the secondary carriers, it is necessary to define a DL-reference UL/DL configuration for the secondary carriers. At this time, in the downlink data channel retransmission timing for the secondary carriers, a downlink retransmission timing mode for the DL-reference UL/DL configuration may be adopted.
As shown in FIG. 1, in the TDD carrier aggregation, the configuration #2 in Table 1 is adopted by the primary carrier, the configuration #0 is adopted by the secondary carriers, and the configuration #2 is defined as the DL-reference UL/DL configuration for the secondary carriers. Correspondingly, depending on the downlink retransmission timing mode for the DL-reference UL/DL configuration (i.e., the configuration #2), in the downlink subframes of the secondary carriers, subframe 2 of the primary carrier may be used to transmit the answer information for the fourth, sixth, seventh and eighth subframes before it. However, among the fourth, sixth, seventh and eighth subframes, the sixth and seventh subframes are downlink subframes. In other words, subframe 2 of the primary carrier is used to transmit the answer information for the sixth and seventh subframes before it. Similarly, in the downlink subframes of the secondary carriers, subframe 7 of the primary carrier may be used to transmit the answer information for the sixth and seventh subframes before it.
A downlink data channel retransmission method for the carrier aggregation defined in the conventional 3GPP LTE protocol merely supports the carrier aggregation in an identical duplexing mode, i.e., it merely supports the TDD+TDD carrier aggregation and the FDD+FDD carrier aggregation. In future, apart from a licensed frequency band, the LTE may also use an unlicensed frequency band (e.g., 2.4 GHz or 5.8 GHz) that has been used by a Wireless Local Area Network (WLAN). The LTE using the unlicensed frequency band may be referred to as LTE-Unlicensed, LTE-U for short. The terminal may use the unlicensed frequency band for the carrier aggregation in the following four ways.
In a first way, the primary carrier is a carrier at the licensed frequency band, and the secondary carriers are carriers at the unlicensed frequency band. In addition, the FDD mode is adopted by the primary carrier, and a Supplement Downlink (SDL) mode is adopted by the secondary carriers, i.e., the unlicensed frequency band is merely used for the downlink transmission.
In a second way, the primary carrier is a carrier at the licensed frequency band, and the secondary carriers are carriers at the unlicensed frequency band. In addition, the FDD mode is adopted by the primary carrier, and the TDD mode is adopted by the secondary carriers.
In a third way, the primary carrier is a carrier at the licensed frequency band, and the secondary carriers are carriers at the unlicensed frequency band. In addition, the TDD mode is adopted by the primary carrier, and the SDL mode is adopted by the secondary carriers, i.e., the unlicensed frequency band is merely used for the downlink transmission.
In a fourth way, the primary carrier is a carrier at the licensed frequency band, and the secondary carriers are carriers at the unlicensed frequency band. In addition, the TDD mode is adopted by the primary carrier, and the TDD mode is adopted by the secondary carriers.
In the above four ways, in order to simplify the implementation of the terminal, the SDL mode may probably be adopted by the secondary carriers, and a TDD+SDL carrier aggregation mode may be used through the combination of the unlicensed frequency band for the secondary carriers and the licensed frequency band for the primary carrier. Currently, the 3GPP protocol has not yet proposed any relevant downlink data channel retransmission method for the TDD (adopted by the primary carrier)+SDL (adopted by the secondary carriers) carrier aggregation mode.